It is known to heat liquid by rotating a rotor in a reservoir of liquid, such an arrangement being shown in my U.S. Pat. No. 4,424,797. It is also known to frictionally heat a liquid byforcing it through restricted orifices such an arrangement being shown in the patent to Horne et al. U.S. Pat. No. 4,344,567.
A problem associated with rotating a rotor in a bath of liquid is that there can be a cavitation problem wherein the liquid periodically separates at the interface between the rotor and liquid. Further, where the heated liquid must be transported to a heat utilization device, such as a heat exchanger separate pump means must usually be provided.